In many systems (as well as non-tabletop games like MMOs) the "weight" of items is a measure of how cumbersome they are, not just weight. Things that make it easier to handle the items might reduce this value.
For example, in Fantasy Flight Games' Star Wars RPG system, items like backpacks and utility belts allow for a larger encumbrance value to be stored inside them than they would total otherwise. (e.g. storing 5 encumbrance of items but only costing 3 points)
In these cases, you could consider this the effect of the items being neatly and compactly stored, making them easier to manage. It would be a lot easier to handle a backpack stuffed with all the items you listed than it would to try and wrangle all of those items loose in your arms.